Image-processing apparatus and image-processing method

ABSTRACT

An image-processing apparatus for processing the image data input from an image-handling device and then outputting the image data to another image-handling device. The apparatus comprises a black-adaptation correction device. The black-adaptation correction device corrects the image data in consideration of the fact that adaptation to black varies from person to person, if the darkest points of the image-handling devices differ from each other. Since the image is so corrected, the colors of the images produced by the image-handling devices look almost the same in spite of the fact that the darkest points of the image-handling devices differ from each other.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for processing the imagedata input from an image-handing device and then outputting the imagedata to another image-handling device. The invention also relates to amethod of processing image data input from an image-handing devicebefore the image data is output to another image-handling device.

As various systems which handle color images come into use in increasingnumbers, it is demanded that color images be reproduced in the same huesby the devices of different types that are used in these systems. Tomeet this demand, image-processing apparatuses have been proposed, eachdesigned to evaluate the devices for their characteristics and adjustthe color values of the image output from the devices to the same value.FIG. 1 shows such an image-processing apparatus. FIG. 2 illustrates theflow of image data in the image-processing apparatus shown in FIG. 1.

Even if the color values of the images output from the devices areadjusted to the same value, however, the colors of the images do notalways appear identical to human eyes. This is because the observationenvironment including ambient light influences the human visual sense.That is to say, the same color looks different in different observationenvironments.

The devices may differ in terms of the darkest points. (That is, theblack part of the image that one device outputs may differ from that ofthe image that another device outputs.) Particularly in this case, theimages output by the devices appear greatly different. Assume that thedarkest point of an input device differs from that of an output device.Then, either graying or black-emphasizing occurs in the image output bythe output device. Consequently, the input image and the output imagemay look quite different in terms of color.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in consideration of the foregoing.The object of the invention is to provide an image-processing apparatusand an image-processing method which process image data so that theimages represented by the data and reproduced by different devices maylook almost identical in terms of color.

According to the first aspect of the present invention, there isprovided an image-processing apparatus for processing the image datainput from an image-handling device and then outputting the image datato another image-handling device. The image-processing apparatuscomprises black-adaptation correction means for correcting the imagedata in consideration of the fact that adaptation to black varies fromperson to person, if the darkest points of the image-handling devicesdiffer from each other, so that the colors of the images produced by theimage-handling devices look almost the same.

In the image-processing apparatus, the black-adaptation correction meanscorrects the image data if the darkest points of the image-handlingdevices differ from each other, in consideration of the fact thatadaptation to black varies from person to person. Hence, the colors ofthe images produced by the image-handling devices look almost the sameeven if the darkest points of the image-handling devices differ fromeach other.

According to the second aspect of the invention, there is provided amethod of processing image data input from an image-handing devicebefore the image data is output to another image-handling device. Themethod comprises the step of: correcting the image data in considerationof the fact that adaptation to black varies from person to person, ifthe darkest points of the image-handling devices differ from each other,so that the colors of the images produced by the image-handling deviceslook almost the same.

In the image-processing method, the image data is corrected if thedarkest points of the image-handling devices differ from each other, inconsideration of the fact that adaptation to black varies from person toperson. Hence, the colors of the images produced by the image-handlingdevices look almost the same even if the darkest points of theimage-handling devices differ from each other.

As mentioned above, in the present invention, image data is corrected ifthe darkest points of image-handling devices differ from each other, inconsideration of the fact that adaptation to black varies from person toperson. Hence, the colors of the images produced by the image-handlingdevices look almost the same even if the darkest points of theimage-handling devices differ from each other. In short, the presentinvention enables the devices of different types to reproduce colorimages that appears identical in color, although the adaptation to blackvaries from person to person

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic representation of a conventional image-processingapparatus;

FIG. 2 is a diagram illustrating the flow of image data in theconventional image-processing apparatus;

FIG. 3 is a diagram showing an image-processing apparatus according tothe present invention;

FIG. 4 is a diagram illustrating the image-processing sequence performedin the image-processing apparatus shown in FIG. 3;

FIG. 5 is a diagram explaining the process of converting an image of thecolor gamut for the input-side device to an image of the color gamut forthe output-side device, through a device optimal color space;

FIG. 6 is a graph representing the relationship between the color gamutof the medium and the black adaptation rate K_(adp);

FIG. 7 is a graph shows the relationship between the Y value (Y_(Op)^(1/3)) present in the device optimal color space and the Y value (Y_(S)^(1/3)) present in the color space which is to be converted to thedevice optimal color space;

FIG. 8 is a graph representing specific examples of the function γ_(Y)(=f(Y_(S,K));

FIG. 9 is a diagram showing another image-processing apparatus accordingto the present invention;

FIG. 10 is a diagram depicting a menu screen designed to set parametersconcerning the environment in which an image is observed;

FIG. 11 is a diagram showing another image-processing apparatusaccording to the present invention;

FIG. 12 is a diagram illustrating an image-processing apparatusaccording to the invention, which is provided in the form of a computersystem.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail, withreference to the accompanying drawings.

In the following description, any image displayed by an image displaydevice shall be called soft copy image, and any image printed on arecording medium such as a paper sheet shall be called hard copy image.In other words, the media for soft copy images are image displaydevices, whereas the media for hard copy images are recording media suchas paper sheets.

1. Structure of the Image-Processing Apparatus

FIG. 3 shows an image-processing apparatus 1 according to thisinvention. The image-processing apparatus 1 receives the image datarepresenting the image displayed by an image display 2 such as a CRTdisplay or a liquid crystal display. The apparatus 1 processes the imagedata and outputs the same to a printer 3.

In this case, the input-side device is the image display 2, and theoutput-side device is the printer.3. According to the present invention,however, the input- and output-side devices are not limited to an imagedisplay and a printer, respectively. For example, the input-side devicemay be an image scanner, a camera or the like, and the output-sidedevice may be an image display or the like. As shown in FIG. 3, theinput- and output-side devices are connected directly to theimage-processing apparatus 1. Instead, these devices may be connected tothe apparatus 1 by a network.

As shown in FIG. 3, the image-processing apparatus 1 comprises aimage-processing section 11, a first input-side sensor 12, a secondinput-side sensor 13, a first output-side sensor 15, and a secondoutput-side sensor 16. The image-processing section 11 processes theimage data received from the image display 2. The first input-sidesensor 12 receives light L₁ emitted from the image display 2, therebydetecting the reflectance and the like of the screen of the imagedisplay 2. The second input-side sensor 13 detects the ambient light L₂existing at the time of observing the image displayed by the imagedisplay 2. The first output-side sensor 15 receives the light L₃ from aprint sheet 14 on which the printer 3 will print the image, therebydetecting the total luminance and the like of the print sheet 14. Thesecond output-side sensor 16 detects the ambient light L₄ existing atthe time of observing the image printed on the print sheet 14 by theprinter 3. The image-processing section 11 comprises an input-sideconverter 21, an input-side observation environment changing circuit 22,an image-editing circuit 23, an output-side observation environmentchanging circuit 24, and an output-side converter 25.

The input-side converter 21 receives the RGB value of the imagedisplayed by the image display 2. Using an input-side device profile,the converter 21 converts the RGB value to an XYZ value, or atristimulus value based on the human visual sense. The input-side deviceprofile is a file that contains conversion formula or a conversion tablefor converting the RGB value received from the image display 2, to a XYZvalue based on the human visual sense. The profile has been prepared onthe basis of the characteristics of the image display 2. The XYZ valueis supplied from the input-side converter 21 to the input-sideobservation environment changing circuit 22.

The input-side observation environment changing circuit 22 a conversionprocess based on a chromatic adaptation model (S-LMS) later described,on the XYZ value received from the input-side converter 21. Further, thecircuit 22 performs a conversion process on the XYZ value so that thevalue may be used in the device optimal color space as will be describedlater Thus, the circuit 22 converts the XYZ value to an XYZ value(X_(OP)Y_(OP)Z_(OP)) for use in a device optimal color space.(Hereinafter, the XYZ value obtained by the conversion will be calledX_(OP)Y_(OP)Z_(OP) value. The X_(OP)Y_(OP)Z_(OP) value is supplied fromthe input-side observation environment changing circuit 22 to theimage-editing circuit 23.

To convert the XYZ value received from the input-side converter 21, theinput-side observation environment changing circuit 22 receives, fromthe first input-side sensor 12, the detection signal representing thereflectance and the like of the screen of the image display 2. Thecircuit 22 also receives, from the second input-side sensor 13, thedetection signal representing the ambient light L₂ that exists at thetime of observing the image displayed by the image display 2. From inthese detection signals the circuit 22 obtains the parameters concerningthe environment in which the image displayed by the image display 2 isobserved. Using the parameters, the circuit 22 performs a conversionprocess based on the chromatic adaptation model and a conversion processfor making it possible to use the XYZ value in the device optimal colorspace.

The image-editing circuit 23 effects an image-editing process, such ascolor gamut compression, on the X_(OP)Y_(OP)Z_(OP) value received fromthe input-side observation environment changing circuit 22. TheX_(OP)Y_(OP)Z_(OP) value, thus processed, is supplied from theimage-editing circuit 23 to the output-side observation environmentchanging circuit 24.

The output-side observation environment changing circuit 24 processesthe X_(OP)Y_(OP)Z_(OP) value, thereby converting the same from one foruse in the device optimum color space to one for use in an LMS colorspace. Further, the circuit 24 performs an inverse conversion processbased on the chromatic adaptation model, which will be described later.Thus, the circuit 24 converts the X_(OP)Y_(OP)Z_(OP) value received fromthe image-editing circuit 23, to an XYZ value, which is a tristimulusvalue, based on the human visual sense.

In order to convert the X_(OP)Y_(OP)Z_(OP) value received from theimage-editing circuit 23, the output-side observation environmentchanging circuit 24 receives the detection signal output from the firstoutput-side sensor 15 and the detection signal output from the secondoutput-side sensor 16. The detection signal output by the sensor 15represents the total luminance and the like of the print sheet 14 onwhich the printer 3 will print the image. The detection signal output bythe sensor 16 represents the ambient light L₄ existing at the time ofobserving the image printed on the print sheet 14 by the printer 3. Fromthese detection signals the circuit 24 obtains parameters concerning theenvironment in which the image printed on the print sheet 14 by theprinter 3 is observed. Using these parameters, the circuit 24 convertsthe X_(OP)Y_(OP)Z_(OP) value, from one for use in the device optimumcolor space to one for use in an LMS color space, and effects conversionprocess based on the chromatic adaptation model.

The output-side converter 25 receives the XYZ value from the output-sideobservation environment changing circuit 24. The output-side converter25 converts the XYZ value to a CMY value (or a CMYK value) that theprinter 3 will used to print the image, by using the an output-sidedevice profile. The output-side device profile is a file that containsconversion formula or a conversion table for converting the XYZ valuebased on the human visual sense to a CMY value, which the printer 3 willuse to print the image. The output-side profile has been prepared on thebasis of the characteristics of the printer 3. The CMY value is suppliedfrom the output-side converter 25 to the printer 3. The printer 3 printsthe image on a print sheet 14.

2. Sequence of Processing Image Data

The above-mentioned sequence of processing image data will be explained,with reference to FIG. 4.

As shown in FIG. 4, the input-side converter 21 performs a conversionprocess based on the input-side device profile, on the RGB value of theimage displayed by the image display 2. The RGB value is therebyconverted to an XYZ value.

Next, the input-side observation environment changing circuit 22subjects the XYZ value to a conversion process based on the chromaticadaptation model. The XYZ value is thereby converted to an LMS value forused in an LMS color space that does not depend on the observationenvironment. (Hereinafter, the LMS value will be called L_(S)M_(S)S_(S)value.)

The input-side observation environment changing circuit 22 converts theL_(S)M_(S)S_(S) value to an X_(OP)Y_(OP)Z_(OP) value for use in thedevice optical color space.

Next, the image-editing circuit 23 subjects the X_(OP)Y_(OP)Z_(OP) valueto an image-editing process, such as color gamut compression. In theimage-editing process, the X_(OP)Y_(OP)Z_(OP) value is converted to anL*a*b* value for use in a sense-equivalent color space. (Hereinafter,the L*a*b* value will be referred to as L_(S)*a_(S)*b_(S)* value.) Thecircuit 23 then performs the image-editing process, such as color gamutcompression, on the L_(S)*a_(S)*b_(S)* value. Further, the circuit 23converts the L_(S)*a_(S)*b_(S)* value, thus processed, to theX_(OP)Y_(OP)Z_(OP) value for use in the device optimal color space.

Then, the output-side observation environment changing circuit 24converts the X_(OP)Y_(OP)Z_(OP) value to a L_(S)M_(S)S_(S) value for usein the LMS color space that does not depend on the observationenvironment

Further, the circuit 24 subjects the L_(S)M_(S)S_(S) value to aconversion process based on the chromatic adaptation model, thusconverting the L_(S)M_(S)S_(S) value to an XYZ value.

Finally, the output-side converter 25 converts the XYZ value to a CMYvalue. The CMY value is output to the printer 3.

In the sequence of processing image data, described above, the dataconverted on the basis of the input-side device profile remains notdepending on the color space of the device until it is subjected to theconversion process based on the chromatic adaptation model. The dataconverted on the basis of the data converted on the basis of theinput-side device profile remains not depending on the color space ofthe device until it is subjected to the conversion process based on thechromatic adaptation model does not depend on the observationenvironment until it is subjected to the inverse conversion processbased on chromatic adaptation model. The data converted for use in thedevice optical color space remains not depending on the dynamic range ofthe device until it is converted to an L_(S)M_(S)S_(S) value for use inthe LMS color space that does not depend on the observation environment.

3. Conversion Process Based on Color Adaptation Model

The conversion process, which the input-side observation environmentchanging circuit 22 performs in the image-processing apparatus 1 on thebasis of the chromatic adaptation model, will be explained in detail. Itshould be noted that the output-side observation environment changingcircuit 24 effects an inverse conversion process based on the chromaticadaptation model, as will be described below.

In an observation environment including ambient light, the color of theimage appears differently to human eyes, because the ambient lightinfluences the human eyes. Hence, a conversion process based on thechromatic adaptation model is carried out to compensate for the changein color of the image viewed in the observation environment thatincludes ambient light. In other words, the conversion process based onthe chromatic adaptation model is a process of compensating forcorrecting the color of the image viewed in the observation environmentthat includes ambient light.

The chromatic adaptation model used here is fundamentally a Von Kriesadaptation model. The model is processed in three major steps. The firststep is correction of contrast, the second step is conversion of atristimulus value to a cone signal, and the third step is correction ofchromatic adaptation. Of these steps, the third is the most important.The third steps consists of two sub-steps. The first sub-step isconsideration of incomplete adaptation, and the second sub-step isconsideration of mixed adaptation. These steps of processing thechromatic adaptation will be explained below, one after another.

In the conversion process based on the chromatic adaptation model andthe conversion process for obtaining values for use in the deviceoptimal color space, both will be described later, some parameters areapplied, which concern with the observation environment in which theimage is looked at. These parameters are obtained from the detectionsignals generated by the first input-side sensor 12, the secondinput-side sensor 13, first output-side sensor 15, and secondoutput-side sensor 16.

More specifically, in the conversion process performed by the input-sideobservation environment changing circuit 22, the parameters concerningthe environment in which the image is observed are obtained from thereflectance and the like of the screen of the image display 2, whichhave been detected by the first input-side sensor 12, and the ambientlight L₂ which exists at the time of observing the image displayed bythe image display 2 and which has been detected by the second input-sidesensor 13. Similarly, in the conversion process effected by theoutput-side observation environment changing circuit 24, the parametersconcerning the environment in which the image is observed are obtainedfrom the total luminance and the like of the print sheet 14, which havebeen detected by the first output-side sensor 15, and the ambient lightL₄ which exists at the time of observing the image printed on the printsheet 14 by the printer 3 and which has been detected by the secondoutput-side sensor 16.

(1) Correction of Contrast

Correction of contrast is first performed in the conversion processbased on the chromatic adaptation model.

If the ambient light has high luminance, the image that the imagedisplay 2 displays will have low contrast. This is because the screen ofthe image display 2 reflects the ambient light, inevitably causingblack-emphasis. Most image displays, such as CRT displays and liquidcrystal displays, have an antireflection film on the screen. Theantireflection film cannot completely prevent the reflection of ambientlight at the screen. As long as the ambient light exists, the blackreproduced on the screen cannot be darker than the black provided by thelight reflected from the screen. As the CIELAB formula teaches, thehuman visual sense is keen to dark colors, and the contrast of the imagedecreases if black is emphasized. Therefore, the light reflected fromthe screen of the image display 2 is taken into consideration toaccomplish the correction of contrast.

First, an offset value corresponding to the light reflected from thescreen is added to the XYZ value that has been obtained by convertingthe RGB value of the image displayed by the image display 2. An X′ Y′ Z′value is thereby obtained as can be understood from the followingequation (1-1). In the equation(1-1), the XYZ value is normalized withY′_(MW) expressed by the following equation(1-2), so that Y′ may acquirethe maximum value of 1. In the equations (1-1) and (1-2), R_(bk) is thereflectance of the screen of the image display (usually, 3 to 5% in thecase of CRT displays), X_(PRD)Y_(PRD)Z_(PRD) is the absolute luminancethat the ambient light has when reflected by a perfect reflectingdiffuser, and Y_(MW) is the absolute luminance of the white point of themedium (i.e., the image display 2). $\begin{matrix}{\begin{bmatrix}\overset{\_}{X^{\prime}} \\\overset{\_}{Y^{\prime}} \\\overset{\_}{Z^{\prime}}\end{bmatrix} = {\frac{1}{Y_{MW}^{\prime}}\{ {\begin{bmatrix}X \\Y \\Z\end{bmatrix} + {R_{bk} \cdot \begin{bmatrix}X_{PRD} \\Y_{PRD} \\Z_{PRD}\end{bmatrix}}} \}\quad{where}}} & ( {1\text{-}1} ) \\{Y_{MW}^{\prime} = {Y_{MW} + {R_{bk} \cdot Y_{PRD}}}} & ( {1\text{-}2} )\end{matrix}$

Since the input-side device is the image display 2, it is desirable toperform the correction of contrast as has been described above. If theinput-side device is an image scanner or the like and if the imagesubjected to the correction of contrast is a hard copy image, it will beunnecessary to carry out the correction of contrast. In such a case, the{overscore (X)}′ {overscore (Y)}′ {overscore (Z)}′ value is normalizedwith Y′_(MW) expressed by the following equation (1-4), therebyobtaining {overscore (X)}′ {overscore (Y)}′ {overscore (Z)}′. In theequation (1-4), Y_(paper) is the absolute luminance at the white pointof the medium. In the case where an image scanner reads the imageprinted on a paper sheet, Y_(paper) is the absolute luminance of thepaper sheet on which the image has been printed. $\begin{matrix}{\begin{bmatrix}\overset{\_}{X^{\prime}} \\\overset{\_}{Y^{\prime}} \\\overset{\_}{Z^{\prime}}\end{bmatrix} = {\begin{bmatrix}{X/Y_{MW}^{\prime}} \\{Y/Y_{MW}^{\prime}} \\{Z/Y_{MW}^{\prime}}\end{bmatrix}\quad{where}}} & ( {1\text{-}3} ) \\{Y_{MW}^{\prime} = Y_{paper}} & ( {1\text{-}4} )\end{matrix}$(2) Conversion of Tristimulus Value (XYZ) to Cone Signal

Next, the tristimulus value (i.e., XYZ value) is converted to a conesignal (LMS value). This conversion process uses a matrix. The matrixformulae that can be applied to the conversion process are as follows.(i) Hunt-Pointer-Estevez Conversion $\begin{matrix}{\begin{bmatrix}\overset{\_}{L} \\\overset{\_}{M} \\\overset{\_}{S}\end{bmatrix} = {\begin{bmatrix}0.3897 & 0.6890 & {- 0.7878} \\{- 0.2298} & 1.1834 & 0.0464 \\0.0 & 0.0 & 1.0000\end{bmatrix}_{E}\begin{bmatrix}\overset{\_}{X^{\prime}} \\\overset{\_}{Y^{\prime}} \\\overset{\_}{Z^{\prime}}\end{bmatrix}}} & ( {1\text{-}5} )\end{matrix}$(ii) Bradford Conversion $\begin{matrix}{\begin{bmatrix}\overset{\_}{L} \\\overset{\_}{M} \\\overset{\_}{S}\end{bmatrix} = {\begin{bmatrix}0.8951 & 2.664 & {- 0.1614} \\{- 0.7502} & 1.7135 & 0.0367 \\0.0389 & {- 0.0685} & 1.0296\end{bmatrix}_{E}\begin{bmatrix}\overset{\_}{X^{\prime}} \\\overset{\_}{Y^{\prime}} \\\overset{\_}{Z^{\prime}}\end{bmatrix}}} & ( {1\text{-}6} )\end{matrix}$(iii) sRGB Conversion $\begin{matrix}{\begin{bmatrix}\overset{\_}{L} \\\overset{\_}{M} \\\overset{\_}{S}\end{bmatrix} = {\begin{bmatrix}3.2406 & {- 1.5372} & {- 0.4986} \\{- 09689} & 1.8758 & 0.0415 \\0.0557 & {- 0.2040} & 1.0570\end{bmatrix}_{E}\begin{bmatrix}\overset{\_}{X^{\prime}} \\\overset{\_}{Y^{\prime}} \\\overset{\_}{Z^{\prime}}\end{bmatrix}}} & ( {1\text{-}7} )\end{matrix}$

The symbol for the sRGB conversion should be sRGB. Nonetheless, LMS isused in the equation (1-7) in order to harmonize the equation (1-7) withthe other equations (1-5) and (1-6).

To convert the tristimulus value:(XYZ value) to a cone signal (LMSvalue), the most appropriate matrix formula is selected from among theabove-mentioned formulae in accordance with the characteristics and thelike of the image, and X′ Y′ Z′ are converted to LMS in accordance withthe matrix formula selected.

The conversion of the tristimulus value (XYZ value) to the cone signal(LMS value) is not absolutely necessary. Rather, this conversion neednot be carried out. If the conversion is not effected, the symbol of X′Y′ Z′ should be used in the equations that will follow. Instead, LMSwill be used in the following equations for harmonization ofdescription, even if this conversion is not performed.

(3) Correction of Chromatic Adaptation

Next, the correction of chromatic adaptation is performed in accordancewith the environment in which the image is observed.

The receptor cells called ones in the human retina have theirsensitivity changed to perceive a light source as being white, in thesame manner as white balance is attained in a video camera That is, thesignals generated by the cones are normalized with the value of thewhite point. To cope with the changes in the sensitivity of the cones,the chromatic adaptation is corrected on the basis of the Von Kriesadaptation model.

The human visual sense is not always fully adapted to the white point ofa light source. Hence, the chromaticity of the light is not used as thewhile point (hereinafter referred to as adaptation white point to whichthe human visual sense is adapted. Rather, in the present model, theadaptation white point is determined from incomplete adaptation andmixed adaptation.

(3.1) Incomplete Adaptation

When we look at the image displayed by the image display 2, our visualsense tries to adapt itself to the white point of the screen. Even if weobserve the image in a dark room, the visual sense cannot be perfectlyadapted to the white point if the white point greatly deviates from thestandard illuminating light D₆₅. Particularly, the more the chromaticityof the white point deviates from that of the standard illuminating lightD₆₅ or the standard illuminating light E, the more incomplete theadaptation is. In addition, the lower the luminance of the white pointis, the more incomplete the adaptation is. In view of the incompleteadaptation, an adaptation white point (hereinafter called incompleteadaptation white point is determined in the image-processing apparatus1.

The incomplete adaptation white point can be determined by, for example,the Hunt, R-LAB system, the Naya system and the CIECAM97s system. Thesesystems will be described below In the image-processing apparatus 1, thesystem, which seems the most appropriate in accordance with thecharacteristics and the like of the image, is selected from thesesystems, and the incomplete adaptation white point is determined by thesystem selected.

In the following equations, {overscore (L)}′_(MW) {overscore (M)}′_(MW){overscore (S)}′_(MW) is the LMS value of the incomplete adaptationwhite point, and {overscore (L)}_(MW) {overscore (M)}_(MW) {overscore(S)}_(MW) is the LMS value obtained by normalizing the tristimulus valueX_(MW) Y_(MW) Z_(MW) of the absolute-luminance of the white point on thescreen. The symbol Y′_(MW) is the absolute luminance [cd/m²] representedby the equation (1-2) or (1-4) described above.(i) Hunt, R-LAB system $\begin{matrix}{\begin{bmatrix}{\overset{\_}{L}}_{MW}^{\prime} \\{\overset{\_}{M}}_{MW}^{\prime} \\{\overset{\_}{S}}_{MW}^{\prime}\end{bmatrix} = {\begin{bmatrix}{1/P_{L}} & 0 & 0 \\0 & {1/P_{M}} & 0 \\0 & 0 & {1/P_{S}}\end{bmatrix}\begin{bmatrix}{\overset{\_}{L}}_{MW} \\{\overset{\_}{M}}_{MW} \\{\overset{\_}{S}}_{MW}\end{bmatrix}}} & ( {1\text{-}8} )\end{matrix}$wherein $\begin{matrix} \begin{matrix}{P_{L} = {( {1 + Y_{MW}^{{\prime 1}/3} + l_{E}} )/( {1 + Y_{MW}^{{\prime 1}/3} + {1/l_{E}}} )}} \\{P_{M} = {( {1 + Y_{MW}^{{\prime 1}/3} + m_{E}} )/( {1 + Y_{MW}^{{\prime 1}/3} + {1/m_{E}}} )}} \\{P_{S} = {( {1 + Y_{MW}^{{\prime 1}/3} + s_{E}} )/( {1 + Y_{MW}^{{\prime 1}/3} + {1/s_{E}}} )}}\end{matrix} \} & ( {1\text{-}9} ) \\ \begin{matrix}{l_{E} = {3 \cdot {{\overset{\_}{L}}_{MW}/( {{\overset{\_}{L}}_{MW} + {\overset{\_}{M}}_{MW} + {\overset{\_}{S}}_{MW}} )}}} \\{m_{E} = {3 \cdot {{\overset{\_}{M}}_{MW}/( {{\overset{\_}{L}}_{MW} + {\overset{\_}{M}}_{MW} + {\overset{\_}{S}}_{MW}} )}}} \\{s_{E} = {3 \cdot {{\overset{\_}{S}}_{MW}/( {{\overset{\_}{L}}_{MW} + {\overset{\_}{M}}_{MW} + {\overset{\_}{S}}_{MW}} )}}}\end{matrix} \} & ( {1\text{-}10} )\end{matrix}$(ii) Naya System $\begin{matrix}{\begin{bmatrix}{\overset{\_}{L}}_{MW}^{\prime} \\{\overset{\_}{M}}_{MW}^{\prime} \\{\overset{\_}{S}}_{MW}^{\prime}\end{bmatrix} = {\begin{bmatrix}{1/P_{L}^{\prime}} & 0 & 0 \\0 & {1/P_{M}^{\prime}} & 0 \\0 & 0 & {1/P_{S}^{\prime}}\end{bmatrix}\begin{bmatrix}{\overset{\_}{L}}_{MW} \\{\overset{\_}{M}}_{MW} \\{\overset{\_}{S}}_{MW}\end{bmatrix}}} & ( {1\text{-}11} )\end{matrix}$where $\begin{matrix} \begin{matrix}{P_{L}^{\prime} = {P_{L} \times k}} \\{P_{M}^{\prime} = {P_{M} \times k}} \\{P_{S}^{\prime} = {P_{S} \times k}} \\{k = {{0.3710( {{\overset{\_}{L}}_{MW}/P_{L}} )} + {0.6291( {{\overset{\_}{M}}_{MW}/P_{M}} )}}}\end{matrix} \} & ( {1\text{-}12} ) \\ \begin{matrix}{P_{L} = {( {1 + Y_{MW}^{{\prime 1}/3} + l_{E}} )/( {1 + Y_{MW}^{{\prime 1}/3} + {1/l_{E}}} )}} \\{P_{M} = {( {1 + Y_{MW}^{{\prime 1}/3} + m_{E}} )/( {1 + Y_{MW}^{{\prime 1}/3} + {1/m_{E}}} )}} \\{P_{S} = {( {1 + Y_{MW}^{{\prime 1}/3} + s_{E}} )/( {1 + Y_{MW}^{{\prime 1}/3} + {1/s_{E}}} )}}\end{matrix} \} & ( {1\text{-}13} ) \\ \begin{matrix}{l_{E} = {3 \cdot {{\overset{\_}{L}}_{MW}/( {{\overset{\_}{L}}_{MW} + {\overset{\_}{M}}_{MW} + {\overset{\_}{S}}_{MW}} )}}} \\{m_{E} = {3 \cdot {{\overset{\_}{M}}_{MW}/( {{\overset{\_}{L}}_{MW} + {\overset{\_}{M}}_{MW} + {\overset{\_}{S}}_{MW}} )}}} \\{s_{E} = {3 \cdot {{\overset{\_}{S}}_{MW}/( {{\overset{\_}{L}}_{MW} + {\overset{\_}{M}}_{MW} + {\overset{\_}{S}}_{MW}} )}}}\end{matrix} \} & ( {1\text{-}14} )\end{matrix}$(iii) System Adopting D Factor Used in CIECAM97s $\begin{matrix} \begin{matrix}{{\overset{\_}{L}}_{MW}^{\prime} = \frac{{\overset{\_}{L}}_{MW}}{D + {{\overset{\_}{L}}_{MW}( {1 - D} )}}} \\{{\overset{\_}{M}}_{MW}^{\prime} = \frac{{\overset{\_}{M}}_{MW}}{D + {{\overset{\_}{M}}_{MW}( {1 - D} )}}} \\{{\overset{\_}{S}}_{MW}^{\prime} = \frac{{\overset{\_}{S}}_{MW}}{D + {{\overset{\_}{S}}_{MW}( {1 - D} )}}}\end{matrix} \} & ( {1\text{-}14} ) \\ \begin{matrix}{D = {F - {F/\lbrack {1 + {2( L_{A}^{1/4} )} + ( {L_{A}^{2}/300} )} \rbrack}}} \\{L_{A} = {Y_{MW}^{\prime}/5}} \\{F = {{const}.}}\end{matrix} \} & ( {1\text{-}15} )\end{matrix}$

In the equation (1-15) presented above, F is a constant that depends onthe observation environment. For example, F=1.9 if the observationenvironment is ordinarily luminous; F=0.9 if the environment is a dim,with a little amount of ambient light; and F=0.9 if the image is printedon a transparent paper sheet. A nonlinear exponential parameter needs tobe provided for {overscore (S)}′_(MW) in the actual CIECAM97s system,but such an exponential parameter is deleted from the equation (1-14)for the sake of simplicity. The exponential parameter may of course betaken into consideration.

If the image display 2 is the input-side device, it is desirable toapply incomplete adaptation. The input-side device may be an imagescanner or the like, and the image processed may be a hard copy image.In this case, it is unnecessary to apply incomplete adaptation, and thevalue of {overscore (L)}′_(MW) {overscore (M)}′_(MW) {overscore(S)}′_(MW) (LMS value), obtained by normalizing the tristimulus value{overscore (L)}_(MW) {overscore (M)}_(MW) {overscore (S)}_(MW) of thewhite point of the medium, is applied without being modified, as can beseen from the following equation (1-16). $\begin{matrix}{\begin{bmatrix}{\overset{\_}{L}}_{M\quad W}^{\prime} \\{\overset{\_}{M}}_{M\quad W}^{\prime} \\{\overset{\_}{S}}_{M\quad W}^{\prime}\end{bmatrix} = \begin{bmatrix}{\overset{\_}{L}}_{M\quad W} \\{\overset{\_}{M}}_{M\quad W} \\{\overset{\_}{S}}_{M\quad W}\end{bmatrix}} & ( {1\text{-}16} )\end{matrix}$(3.2) Mixed Adaptation

The image displayed by the image display 2, i.e., a soft copy image, isscarcely observed in a dark room. In most offices, the image is observedunder fluorescent lamps that have a correlation color temperature (ACT)of about 4150K. On the other hand, the correlation color temperature ofthe white point of CRT displays widely as image displays isapproximately 9300K. In the case of the soft copy image, the white pointof the medium .(i.e., image display 2) usually has a color temperaturethat differs from the color temperature of the ambient light.

In the case of the image printed on a recording medium (i.e., a hardcopy image), the recording medium is usually a white paper sheet. Theimage may be printed on a yellowish paper sheet, such as newspapersheets. If the recording medium is not perfectly white, the white pointof the recording medium differs from the white of the ambient light.That is, in the case of a hard copy image, too, the white point of therecording medium (i.e., a paper sheet or the like) may differ from thecolor temperature of the ambient light in some cases.

In both the soft copy image and the hard copy image, the white point ofthe medium may differ from the color temperature of the ambient light.If so, the human visual sense is regarded as partly adapted to both thewhite point and the color temperature. Let us assume, therefore, thatthe white point to which our visual sense is adapted lies some wherebetween the white point of the medium and the color temperature of theambient light Then, the LMS value (i.e., {overscore (L)}″_(MW){overscore (M)}″_(MW) {overscore (S)}″_(MW)) of the white point to whichour visual sense is actually adapted is defined by the followingequations (1-17) and (1-18), where R_(adp) is the adaptation ratio atwhich the human visual sense is adapted to the white of the medium.$\begin{matrix} \begin{matrix}\begin{matrix}{{\overset{\_}{L}}_{M\quad W}^{''} = {{R_{adp} \cdot \lbrack \frac{Y_{M\quad W}^{\prime}}{Y_{adp}} \rbrack^{1/3} \cdot {\overset{\_}{L}}_{M\quad W}^{\prime}} + {( {1 - R_{adp}} ) \cdot \lbrack \frac{Y_{PRD}}{Y_{adp}} \rbrack^{1/3} \cdot {\overset{\_}{L}}_{PRD}}}} \\{{\overset{\_}{M}}_{M\quad W}^{''} = {{R_{adp} \cdot \lbrack \frac{Y_{M\quad W}^{\prime}}{Y_{adp}} \rbrack^{1/3} \cdot {\overset{\_}{M}}_{M\quad W}^{\prime}} + {( {1 - R_{adp}} ) \cdot \lbrack \frac{Y_{PRD}}{Y_{adp}} \rbrack^{1/3} \cdot {\overset{\_}{M}}_{PRD}}}}\end{matrix} \\{{\overset{\_}{S}}_{M\quad W}^{''} = {{R_{adp} \cdot \lbrack \frac{Y_{M\quad W}^{\prime}}{Y_{adp}} \rbrack^{1/3} \cdot {\overset{\_}{S}}_{M\quad W}^{\prime}} + {( {1 - R_{adp}} ) \cdot \lbrack \frac{Y_{PRD}}{Y_{adp}} \rbrack^{1/3} \cdot {\overset{\_}{S}}_{PRD}}}}\end{matrix} \} & ( {1\text{-}17} ) \\{Y_{adp} = \{ {{R_{adp} \cdot Y_{M\quad W}^{{\prime 1}/3}} + {( {1 - R_{adp}} ) \cdot Y_{PRD}^{{\prime 1}/3}}} \}^{3}} & ( {1\text{-}18} )\end{matrix}$

In the equations (1-17) and (1-18), {overscore (L)}_(PRD) {overscore(M)}_(PRD) {overscore (S)}_(PRD) is the LSM value that has been obtainedby normalizing the tristimulus value X_(PRD) Y_(PRD) Z_(PRD) of theabsolute luminance of the ambient light reflected from a perfectreflecting diffuser. Symbol Y′_(MW) is the absolute luminance of thewhite point of the medium. (The absolute luminance is one evaluated inconsideration of the light reflected from the screen, if the medium isthe image display 2). If the medium is the image display 2, Y′_(MW) isthe absolute luminance of the white point of the screen of the display2. If the medium is a white paper sheet, Y′_(MW) is the absoluteluminance of the paper sheet.

Introduced into the equations (1-17) and (1-18) are weightingcoefficients (Y′_(NW)/Y_(adp))^(1/3) and (Y′_(PRD)/Y_(adp))^(1/3). Thesecoefficients are applied when the absolute luminance of the ambientlight reflected from the perfect reflecting diffuser differs from theabsolute luminance of the white point of the medium.

In the equations (1-17) and (1-18) presented above, it is desired thatthe adaptation ratio R_(adp) be about 0.4 to 0.7 in normal environment.More precisely, is set to 0.6. The adaptation ratio R_(adp) of 1.0 meansthat the human visual sense is perfectly (100%) adapted to the medium,not influenced by the ambient light at all. In other words, the ratioR_(adp) is 1.0 when the human visual sense is adapted 100% to the imagedisplay 2, not influenced by the ambient light, if the medium used isthe image display 2, or when the visual sense is adapted 100% to therecording medium, not influenced by the ambient light, if the recordingmedium used is, for example, a paper sheet. Conceptually, the ratioR_(adp) of 1.0 is achieved when CIELAB is adjusted to the human visualsense. The value of 0.0 is attained when the human visual sense isadapted 100% to the ambient light. This is equivalent to the CIE/XYZadjusted to the human visual sense.

(3.3) Correction of Color Adaptation, Based on Von Kries Model

The adaptation white point ({overscore (L)}″_(MW) {overscore (M)}″_(MW){overscore (S)}″_(MW)) that has been obtained in consideration of theincomplete adaptation and mixed adaptation is substituted in the VonKries adaptation rule, as indicated in the following equation (1-19).The color adaptation is thereby corrected, providing an LMS value(L_(S)M_(S)S_(S)) that does not depend on the observation environment.In the equation (1-19), LMS in the right side is a value obtained in thesecond step of converting a tristimulus value to a cone signal, whichhas been described above. $\begin{matrix}{\begin{bmatrix}{Ls} \\{Ms} \\{Ss}\end{bmatrix} = {\begin{bmatrix}{1/{\overset{\_}{L}}_{M\quad W}^{''}} & 0 & 0 \\0 & {1/{\overset{\_}{M}}_{M\quad W}^{''}} & 0 \\0 & 0 & {1/{\overset{\_}{S}}_{M\quad W}^{''}}\end{bmatrix}\begin{bmatrix}\overset{\_}{L} \\\overset{\_}{M} \\\overset{\_}{S}\end{bmatrix}}} & ( {1\text{-}19} )\end{matrix}$

The input-side observation environment changing circuit 22 of theimage-processing apparatus 1 performs the conversion process based onthe chromatic adaptation model Thus, the circuit 22 converts the XYZvalue received from the input-side converter 21, to an LMS value(L_(S)M_(S)S_(S)) for the LMS color space that does not depend on theobservation environment. The output-side observation environmentchanging circuit 24 of the image-processing apparatus 1 effects theinverse conversion process based on the chromatic adaptation model.

4. Conversion Process to the Device Optical Color Space

The conversion process to the device optimal color space, performed bythe input-side observation environment changing circuit 22, will bedescribed in detail.

The conversion process to the device optimal color space is carried outin consideration of black-adaptation correction. The black-adaptationcorrection is effected so that a color looks almost the same onwhichever medium, even if the darkest points of the media have differenttristimulus values. Suppose an image is merely converted from the colorgamut of a medium to the color gamut of another medium. Then, the colorof the image appears different to the human eyes if the darkest pointsof the media have different tristimulus values. This is inevitablybecause the adaptation to black varies from person to person. This iswhy the black-adaptation correction is performed, whereby the colorlooks almost the same on whichever medium even if the darkest points ofthese media have different tristimulus values.

As shown in FIG. 5, the color space is changed to the device optimalcolor space, thus effecting the black-adaptation correction, in order toconverts the image from the color gamut of the input-side device to thecolor gamut of the output-side device. Ambient light, if any, influencesthe human visual sense. Hence, the color gamut S1 the image has whenambient light exists differs from the color gamut S2 the image has whenno ambient light exists.

As will be explained below, the black-adaptation correction is carriedout by using the XYZ value of the darkest point on the medium (i.e., theXYZ value of the black point).and the XYZ value of the most luminouspoint on the medium (i.e., the XYZ value of the white point).

The conversion process to the device optimal color space is performed inconsideration of black-adaptation correction, in the following manner.First, the LMS value (L_(S)M_(S)S_(S)) defined by the equation (1-19) isconverted to a value (X_(S)Y_(S)Z_(S)) for use in the XYZ color space,as follows: $\begin{matrix}{\begin{bmatrix}{Xs} \\{Ys} \\{Zs}\end{bmatrix} = {\begin{bmatrix}1.9102 & {- 1.1122} & 0.2019 \\0.3709 & 0.6291 & 0.0000 \\0.0 & 0.0 & 1.0000\end{bmatrix}_{E}\begin{bmatrix}{Ls} \\{Ms} \\{Ss}\end{bmatrix}}} & ( {2\text{-}1} )\end{matrix}$

The XYZ value (X_(S,K)Y_(S,K)Z_(S,K)) of the black point that man mayperceive as being darkest is defined by the following equation (2-2).This if the black-adaptation correction. $\begin{matrix} \begin{matrix}{X_{S,K}^{1/3} = {{K_{adp} \cdot X_{S,{MK}}^{1/3}} + {( {1 - K_{adp}} ) \cdot X_{PK}^{1/3}}}} \\{Y_{S,K}^{1/3} = {{K_{adp} \cdot Y_{S,{MK}}^{1/3}} + {( {1 - K_{adp}} ) \cdot Y_{PK}^{1/3}}}} \\{Z_{S,K}^{1/3} = {{K_{adp} \cdot Z_{S,{MK}}^{1/3}} + {( {1 - K_{adp}} ) \cdot Z_{PK}^{1/3}}}}\end{matrix} \} & ( {2\text{-}2} )\end{matrix}$

In the equation (2-2), X_(S,MK)Y_(S,MK)Z_(S,MK) is the tristimulus valuefor the black point of the medium and X_(PK)Y_(PK)Z_(PK) is thetristimulus value for the light reflected from the perfectly black partThese values must be 0 in ideal condition. Symbol K_(adp) is the rate ofadaptation to the human eyes and ranges from 0 to 1. FIG. 6 shows therelationship between the color gamut of the medium and the rate K_(adp)of adaptation to black The adaptation rate K_(adp) having thisrelationship with the color gamut of the medium is applied,accomplishing the black-adaptation correction.

As mentioned above, X_(PK)Y_(PK)Z_(PK) is the tristimulus value for thelight reflected from the perfectly black part and must be 0 in the idealcondition. Hence, we can have the following equation (2-3):X PK =Y PK =Z PK =O  (2-3)

Therefore, the equation (2-2) reduces to the following equation (2-4):$\begin{matrix} \begin{matrix}\begin{matrix}{X_{S,K}^{1/3} = {K_{adp} \cdot X_{S,{MK}}^{1/3}}} \\{Y_{S,K}^{1/3} = {K_{adp} \cdot Y_{S,{MK}}^{1/3}}}\end{matrix} \\{Z_{S,K}^{1/3} = {K_{adp} \cdot Z_{S,{MK}}^{1/3}}}\end{matrix} \} & ( {2\text{-}4} )\end{matrix}$

Next, the XYZ value (X_(S,K)Y_(S,K)Z_(S,K)) of the black point, whichhas been subjected to the black-adaptation correction, is applied todefine the device optimal color space that is a color spacecorresponding to the dynamic range of the device. More specifically, thedevice optimal color space is defined as illustrated by the followingequation (2-5). That is, the XYZ value X_(S,K)Y_(S,K)Z_(S,K)) of theblack point, which has been subjected to the black-adaptationcorrection, is combined with the XYZ value (X=Y=Z=1) for the adaptationwhite point and is thereby defined as an exponential function. Inaccordance with the equation (2-5) the XYZ value (X_(S)Y_(S)Z_(S))obtained by the process of the equation (2-1) is converted to an XYZvalue (X_(OP)Y_(OP)Z_(OP)) in the device optimal color space thatcorresponds to the dynamic range of the device. $\begin{matrix} \begin{matrix}\begin{matrix}{X_{op}^{1/3} = \lbrack \frac{( X_{S} )^{1/3} - ( X_{S,K} )^{1/3}}{1 - ( X_{S,K} )^{1/3}} \rbrack^{\gamma_{X}}} \\{Y_{op}^{1/3} = \lbrack \frac{( Y_{S} )^{1/3} - ( Y_{S,K} )^{1/3}}{1 - ( Y_{S,K} )^{1/3}} \rbrack^{\gamma_{Y}}}\end{matrix} \\{Z_{op}^{1/3} = \lbrack \frac{( Z_{S} )^{1/3} - ( Z_{S,K} )^{1/3}}{1 - ( Z_{S,K} )^{1/3}} \rbrack^{\gamma_{Z}}}\end{matrix} \} & ( {2\text{-}5} )\end{matrix}$

FIG. 7 is a graph showing an example of the function expressed by theabove equation (2-5). In FIG. 7, the Y value (Y_(OP) ^(1/3)) present inthe device optimal, color space is plotted on the ordinate, and the Yvalue (Y_(S) ^(1/3)) present in the color space to be converted to thedevice optimal color space is plotted on the abscissa. As FIG. 7 shows,the device optimal color space is only a fraction of the dynamic rangeif K_(adp)=0 (that is, if no black-adaptation correction is carriedout). This space is one much contracted. If K_(adp) is greater than 0(K_(adp)<0), the device optimal color space expands. In particular, thedevice optimal color space becomes a space fully corresponding to theblack adaptation when K_(adp)=1.0; it becomes a space that utilizes theentire dynamic range of the device.

The exponential parameters γ_(x), γ_(y) and γ_(z) in the equation (2-5)are functions of X_(S,K), Y_(S,K) and Z_(S,K), as is shown in thefollowing equations (2-6): $\begin{matrix} \begin{matrix}{\gamma_{X} = {f( X_{S,K} )}} \\{\gamma_{Y} = {f( Y_{S,K} )}} \\{\gamma_{Z} = {f( Z_{S,K} )}}\end{matrix} \} & ( {2\text{-}6} )\end{matrix}$

The results of the experiment the inventor conducted show that theseexponential parameters γ_(x), γ_(y) and γ_(z) should better be 1 whenX_(S,k), Y_(S,K) and Z_(S,K) is 0, and should better be greater than 1and undergo simple increase when X_(S,K), Y_(S,K) and Z_(S,K) is greaterthan 0. That is, it is desired that these parameters γ_(x), γ_(y) andγ_(z) be defined as functions, each assuming the value of 1 whenX_(S,K), Y_(S,K) and Z_(S,K) is 0, and assuming the value greater than 1and simply increasing when X_(S,K), Y_(S,K) and Z_(S,K) is greater than0. Specific examples of such a function are shown in FIG. 8. Moreprecisely, FIG. 8 is a graph representing two examples A1 and A2 of thefunction f of the exponential parameter, i.e., γ_(y)=f(Y_(S,K)).

In the image-processing apparatus 1, the input-side observationenvironment changing circuit 22 performs the conversion process to thedevice optimal color space in consideration of the black-adaptationcorrection. Thus, the LMS value (L_(S)M_(S)S_(S)) obtained in theconversion process based on the color adaptation model is converted toan XYZ value (X_(OP)Y_(OP)Z_(OP)) for use in the device optimal colorspace that corresponds to the dynamic range of the device.

5. Process after Conversion to Device Optimal Color Space

The XYZ value (X_(OP)Y_(OP)Z_(OP)) obtained in the conversion process tothe device optimal color space is supplied to the image-editing circuit23. The image-editing circuit 23 performs an image-editing process, suchas color gamut compression. In the image-editing process, theX_(OP)Y_(OP)Z_(OP) value is converted to an L_(S)*a_(S)*b_(S)* value.This conversion is expressed by the following equations (2-7):$\begin{matrix} \begin{matrix}{L_{S}^{*} = {116 \cdot ( Y_{OP} )^{1/3} \cdot 16}} \\{a_{S}^{*} = {500 \cdot \{ {( X_{OP} )^{1/3} \cdot ( Y_{OP} )^{1/3}} \}}} \\{b_{S}^{*} = {200 \cdot \{ {( Y_{OP} )^{1/3} \cdot ( Z_{OP} )^{1/3}} \}}} \\{C_{S}^{*} = \sqrt{( a_{s}^{*} )^{2} + ( b_{s}^{*} )^{2}}} \\{{h\quad s^{*}} = {\tan - {1( \frac{b_{s}^{*}}{a_{s}^{*}} )}}}\end{matrix} \} & ( {2\text{-}7} )\end{matrix}$

Then, the image-editing circuit 23 performs an image-editing process,such as color gamut compression, on the L_(S)*a_(S)*b_(S)* value thusobtained Thereafter, the image-editing circuit 23 converts theL_(S)*a_(S)*b_(S)* value to the X_(OP)Y_(OP)Z_(OP) value. TheX_(OP)Y_(OP)Z_(OP) value is supplied to the output-side observationenvironment changing circuit 24. The image-editing process is notabsolutely necessary in the present invention; it may not be carriedout.

The X_(OP)Y_(OP)Z_(OP) value is supplied to from the output-sideobservation environment changing circuit 24 to the output-side devicethrough the output-side converter 25. The output-side converter 25 needsto have a color space optimal to it. This is because theX_(OP)Y_(OP)Z_(OP) value must pass through an optimal color space beforesupplied to the output-side device. The optimal color space of theoutput-side device is identical to the optimal color space of theinput-side device. Therefore, the following equations (2-8) hold true.Suffix IN to some symbols in the equations (2-8) et. seq. indicates thatthe values pertain to the input-side device, while suffix OUT to someother symbols means that the values pertain to the output-side device.$\begin{matrix} \begin{matrix}\begin{matrix}{( X_{{OUT},{OP}} )^{1/3} = ( X_{{IN},{OP}} )^{1/3}} \\{( Y_{{OUT},{OP}} )^{1/3} = ( Y_{{IN},{OP}} )^{1/3}}\end{matrix} \\{( Z_{{OUT},{OP}} )^{1/3} = ( Z_{{IN},{OP}} )^{1/3}}\end{matrix} \} & ( {2\text{-}8} )\end{matrix}$

The device optimal color spaces can be given by the equations (2-5)shown above. The equations (2-5) can be replaced by the followingequations (2-9): $\begin{matrix} \begin{matrix}{\lbrack \frac{( X_{S,{OUT}} )^{\frac{1}{3}} - ( X_{S,{OUT},K} )^{\frac{1}{3}}}{1 - ( X_{S,{OUT},K} )^{\frac{1}{3}}} \rbrack^{{\gamma\quad X},{OUT}} = \lbrack \frac{( X_{S,{IN}} )^{\frac{1}{3}} - ( X_{S,{IN},K} )^{\frac{1}{3}}}{1 - ( X_{S,{IN},K} )^{\frac{1}{3}}} \rbrack^{{\gamma\quad X},{IN}}} \\{\lbrack \frac{( Y_{S,{OUT}} )^{\frac{1}{3}} - ( Y_{S,{OUT},K} )^{\frac{1}{3}}}{1 - ( Y_{S,{OUT},K} )^{\frac{1}{3}}} \rbrack^{{\gamma\quad Y},{OUT}} = \lbrack \frac{( Y_{S,{IN}} )^{\frac{1}{3}} - ( Y_{S,{IN},K} )^{\frac{1}{3}}}{1 - ( Y_{S,{IN},K} )^{\frac{1}{3}}} \rbrack^{{\gamma\quad Y},{IN}}} \\{\lbrack \frac{( Z_{S,{OUT}} )^{\frac{1}{3}} - ( Z_{S,{OUT},K} )^{\frac{1}{3}}}{1 - ( Z_{S,{OUT},K} )^{\frac{1}{3}}} \rbrack^{{\gamma\quad Z},{OUT}} = \lbrack \frac{( Z_{S,{IN}} )^{\frac{1}{3}} - ( Z_{S,{IN},K} )^{\frac{1}{3}}}{1 - ( Z_{S,{IN},K} )^{\frac{1}{3}}} \rbrack^{{\gamma\quad Z},{IN}}}\end{matrix} \} & ( {2\text{-}9} )\end{matrix}$ $\begin{matrix} \begin{matrix}{( X_{S,{OUT}} )^{\frac{1}{3}} = {( {1 - ( X_{S,{OUT},K} )^{\frac{1}{3}}} ) \cdot}} \\{{~~~~~~~~~~~~~~~~~~~~~~~~~~~~~}{\lbrack \frac{( X_{S,{IN}} )^{\frac{1}{3}} - ( X_{S,{IN},K} )^{\frac{1}{3}}}{1 - ( X_{S,{IN},K} )^{\frac{1}{3}}} \rbrack^{\gamma_{X,{IN}}/\gamma_{X,{OUT}}} +}} \\{( X_{S,{OUT},K} )^{\frac{1}{3}}} \\{( Y_{S,{OUT}} )^{\frac{1}{3}} = {( {1 - ( Y_{S,{OUT},K} )^{\frac{1}{3}}} ) \cdot}} \\{\lbrack \frac{( Y_{S,{IN}} )^{\frac{1}{3}} - ( Y_{S,{IN},K} )^{\frac{1}{3}}}{1 - ( Y_{S,{IN},K} )^{\frac{1}{3}}} \rbrack^{\gamma_{Y,{IN}}/\gamma_{Y,{OUT}}} +} \\{( Y_{S,{OUT},K} )^{\frac{1}{3}}} \\{( Z_{S,{OUT}} )^{\frac{1}{3}} = {( {1 - ( Z_{S,{OUT},K} )^{\frac{1}{3}}} ) \cdot}} \\{\lbrack \frac{( Z_{S,{IN}} )^{\frac{1}{3}} - ( Z_{S,{IN},K} )^{\frac{1}{3}}}{1 - ( Z_{S,{IN},K} )^{\frac{1}{3}}} \rbrack^{\gamma_{Z,{IN}}/\gamma_{Z,{OUT}}} +} \\{( Z_{S,{OUT},K} )^{\frac{1}{3}}}\end{matrix} \} & ( {2\text{-}10} )\end{matrix}$

From the equations (2-10) we can find the X_(S)Y_(S)Z_(S) value for theoutput side. The inverse conversion represented by the equation (2-1) iseffected, converting the X_(S)Y_(S)Z_(S) value to the L_(S)M_(S)S_(S)value for use in the LMS color space that does not depend on theobservation environment Thereafter, the inverse conversion process basedon the color adaptation model is performed on the L_(S)M_(S)S_(S) value,thereby converting the L_(S)M_(S)S_(S) value to the XYZ value. Theoutput-side observation environment changing circuit 24 supplies the XYZvalue to the output-side converter 25.

The output-side converter 25 converts the XYZ value supplied from thecircuit 24, to a CMY value on the basis of the output-side deviceprofile. The output-side converter 25 outputs the CMY value to theprinter 3.

As described above, the black-adaptation correction is effected theadaptation to black varies from person to person. As a result, thecolors of the image appears almost the same in both the input-sidedevice and the output-side device, even if the darkest points of theinput- and output-side devices differ from each other. Thus, in theimage-processing apparatus 1 described above, the image displayed by theimage display 2 and the image printed by the printer 3 which hasreceived the data representing the image produced by the display 2 looksalmost the same, though the darkest points of the image display 2 andthe printer 3 differ from each other.

6. Other Embodiments

In the embodiment shown in FIG. 3, the first and second input-sidesensors 12 and 13 and the first and second output-side sensors 15 and 16are used to obtain the parameters concerning the observationenvironment, which are applied in the image-converting process. Theseparameters may be input directly to the image-processing apparatus 1.

FIG. 9 shows an image-processing apparatus 51 that directly receives theparameters concerning the environment in which the image is observed.The components of the apparatus 51 shown in FIG. 9, which are similar tothose of the image-processing apparatus 1 shown in FIG. 3 are designatedat the same reference numerals.

As is illustrated in FIG. 9, the image-processing apparatus 51 comprisestwo parameter-setting circuits 52 and 53, in place of the sensors 12,13, 15 and 16 used in the image-processing apparatus 1 of FIG. 3. Theparameter-setting circuit 52 is connected to the input-side observationenvironment changing circuit 22, and the parameter-setting circuit 53 tothe output-side observation environment changing circuit 24.

In the image-processing apparatus 51, the parameters that the circuit 22needs to convert image data are input to the circuit 22 via theparameter-setting circuit 52 connected to the input-side observationenvironment changing circuit 22. Using the parameters thus input, theinput-side observation environment changing circuit 22 performs theconversion of image data. Further, the parameters that the circuit 24needs to convert image data are input to the circuit 24 via theparameter-setting circuit 53 connected to the output-side observationenvironment changing circuit 24. Using these parameters, the output-sideobservation environment changing circuit 24 performs the conversion ofimage data.

To input the parameters concerning the observation environment from theparameter-setting circuits 52 and 53, it is desired that a menu screenbe used, which is, for example, such a graphical user interface as isillustrated in FIG. 10.

The screen menu shown in FIG. 10 is one used to cause theparameter-setting circuit 52 to set parameters. The screen menu isdesigned, enabling the user of the apparatus 51 to select anindoor-light chromaticity (light source), an indoor-light luminance(surround luminance) and a luminance for the image display 2 (monitorluminance), each from a plurality of choices. In the menu screen shownin FIG. 10, “F6”, “Dark” and “Mid” are selected for the indoor-lightchromaticity, indoor-light luminance and luminance for the image display2, respectively.

The parameter-setting circuit 52 stores the parameters about theobservation environment which correspond to these various choices. Theparameter-setting circuit 52 reads the parameters that correspond to thechoices selected and supplies these parameters to the input-sideobservation environment changing circuit 22. In the case of the menuscreen shown in FIG. 10, the parameters corresponding to the “F6”,“Dark” and “Mid” are selected for the indoor-light chromaticity 6, theindoor-light luminance “Dark”, and the image display luminance “Mid” aresupplied to the input-side observation environment changing circuit 22.The circuit 22 converts the image data in accordance with theseparameters.

In the image-processing apparatus 1 of FIG. 3, the input-side device andthe output-side device are the image display 2 and the printer 3,respectively. The input- and output-side devices are not limited tothese For example, the input-side device and the output-side device maybe an image scanner 62 and an image display 63, as in theimage-processing apparatus 61 illustrated in FIG. 11. The components ofthis apparatus 61, which are similar to those of the apparatus 1 shownin FIG. 3, are designated at the same reference numerals as in FIG. 3.

In the image-processing apparatus 61 of FIG. 11, the first input-sidesensor 12 detects the light L₅ reflected from the paper sheet 64 onwhich the image has been printed to be read by the image scanner 62.From the light L₅ the sensor 12 determines the total luminance of theprinted paper sheet 64. The second input-side sensor 13 detects theambient light L₆ existing when the image printed on the paper sheet 64is observed. The first output-side sensor 15 detects the light L₇ fromthe image display 63 and determines therefrom the reflectance and thelike of the screen of the image display 63. The second output-sidesensor 16 detects the ambient light L₈ existing when the image displayedby the image display 63 is observed.

The present invention is not to the embodiments depicted in FIGS. 3 and11. Rather, the invention can be applied to such a computer system 71 asis illustrated in FIG. 12.

The computer system 71 of FIG. 12 comprises a central processing unit(CPU) 72, a system controller 73, a cache memory 74, a random-accessmemory 75, an external storage control section 76, an input controlsection 77, an image data input/output control section, 78, a videocontroller 79, a sensor control section 80, and a communication controlsection 81. These components are connected to a bus 82.

The external storage control section 76 serves as an interface with anexternal storage device. The section 76 is connected to, for example, ahard disk drive 83 or a CD-ROM drive 84.

The input control section 77 functions as an interface with an inputdevice. The section 77 is connected to, for example, a keyboard 85, amouse-type pointing device 86.

The image data input/output control section 78 is provided as aninterface with a device which handles image data. The section 78 isconnected to, for example, an image scanner 87 or a printer 88.

The video controller 79 serves as an interface with an image display. Itis connected to, for example, a CRT display 89.

The sensor control section 80 operates as an interface with externalsensors. The section 80 is connected, for example, the first and secondinput-side sensors 12 and 13 and the first and second output-sidesensors 15 and 16, all identical to their counterparts of theimage-processing apparatus 1.

The communication control section 81 functions as an interface with acommunication apparatus. The section 81 is connected to, for example, amodem 91 or a hub 92. The computer system 71 can therefore be connectedto the telephone line by the modem 91 that is connected to thecommunication control section 81. Alternatively, the computer system 71can be connected to a predetermined network by the hub 92 that isconnected to the communication control section 81.

In the computer system 71, the CPU 72 processes image data in the sameway as the image-processing section 11 does in the image-processingapparatus 1, under the control of the system controller 73 with theassistance of the cache memory 74 and random-access memory 75.

Hence, the computer system 71 receives the image data from theinput-side device (e.g., image scanner 87 or the like) by the use of theimage data input/output control section 78 or the video controller 79.In the system 71 the CPU 72 processes the image data in the same way asthe image-processing section 11 does in the image-processing apparatus1. The image data thus processed is output to the output-side device(e.g., printer 88, CRT display 89, or the like) through the image datainput/output control section 78 or the video controller 79.

The black-adaptation correction is effected in the computer system as inthe image-processing apparatus 1 when the CPU 72 processes the imagedata, because the adaptation to black varies from person to person As aresult, the colors of the images produced by the input- and output-sidedevices look almost the same, even if the darkest points of the input-and output-side devices differ from each other.

1. An image-processing apparatus for processing the image data inputfrom an image-handling device and then outputting the image data toanother image-handling device, said apparatus comprising:black-adaptation correction means for correcting the image data inconsideration of the fact that adaptation to black varies from person toperson, if the darkest points of the image-handling devices differ fromeach other, so that the colors of the images produced by theimage-handling devices look almost the same.
 2. A method of processingimage data input from an image-handing device before the image data isoutput to another image-handling device, said method comprising the stepof: correcting the image data in consideration of the fact thatadaptation to black varies from person to person, if the darkest pointsof the image-handling devices differ from each other, so that the colorsof the images produced by the image-handling devices look almost thesame.